Extending the life of my PC

I was considering the purchase of a new PC, but my "want" genes far out weigh my "need" genes so the cost of my proposed new PC is more like the GDP of a small nation.

So, I decided to do a basic upgrade instead. My current system is an Intel Mobo, Core2 Duo E6550 CPU, Nvidia 8500GT GPU, 4 GB Ram and 500GB HDD. All running on Vista 32. I decided to upgrade the CPU, add another 4GB of Ram and add a second HDD. I will then upgrade to Win 7 64.

My Query is this, should I go for the E8400 Core2 Duo or go for one of the socket 775 Core2 Quad CPUs? I thought of the Q9650 but it is about double the price of the E8400.